This invention is directed to a glass fiber forming bushing assembly. More specifically, the present invention is directed toward improving the uniformity of temperature conditions within such a bushing, which in turn improves the uniformity of fibers produced by the bushing.
In a typical direct melt forming operation, glass batch materials are heated in a large melting furnace and fed via a refiner to an elongated refractory-lined forehearth having a plurality of bushing assemblies connected to apertures in the bottom of the forehearth. Alternatively, molten glass may be supplied to a bushing by remelting glass agglomerates (e.g., "marbles"). The present invention has applicability to either type of operation. Glass fibers are formed at the bushing by attenuating streams of molten glass through orifices or nozzles ("tips") at the bottom of each bushing assembly.
In the formation of continuous glass fibers, various bushing support structures have been described in the prior art. Commonly, the bushing is supported against the underside of a source of molten glass by a separate frame. Because the bushing is usually heated electrically, the bushing is electrically insulated from the frame. The bushing is set into the frame by means of a castable refractory material that is poured into the space between the bushing and the frame before the assembly is mounted in the fiber forming station. The cast refractory surrounding the bushing also serves the important function of insulating the bushing against heat loss. Maintaining the rheology of the molten glass substantially uniform in all portions of the bushing has been an objective toward which various measures have been directed in the prior art for the sake of promoting unbroken production of filaments having uniform diameters from all portions of the bushing. Therefore, the industry has sought ways to minimize heat loss from the bushing and to maintain temperatures as uniform as possible at each of the filament forming orifices. In some cases, these efforts have not been as effective as would be desired.
In the fiber glass industry, it has been known to use bushings whose side walls taper outwardly toward the bottom. Such a design is useful for maximizing the number of fiber forming orifices on a bushing while minimizing the size of the opening in the bottom of the forehearth. U.S. Pat. No. 4,330,311 shows a typical bushing arrangement of that type. The combination of angled bushing walls with vertical frame members creates a wedge shaped mass of refractory around the bushing, with the thickness of the refractory, and therefore its insulating value, being greater at the top of the bushing, i.e., near the forehearth, relative to that near the bottom of the bushing. It has now been found that the non-uniform refractory thickness associated with this geometry results in localized heat transfer near the bottom of the bushing which has a significant negative effect on the uniformity of fiber forming conditions across the bushing. Additionally, in order to support the cast refractory vertically, the prior art, as typified by the aforesaid patent, sometimes placed a horizontal, inwardly extending flange at the bottom of the frame. This further exacerbates the non-uniform heat loss in the peripheral areas at the bottom of the bushing. It would be desirable if this thermal non-uniformity could be reduced without compromising the structural integrity of the mounting means for the bushing.